NAME

     brk, sbrk — change data segment size

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <unistd.h>

     int
     brk(const void *addr);

     void *
     sbrk(intptr_t incr);

DESCRIPTION

     The brk() and sbrk() functions are legacy interfaces from before the advent of modern
     virtual memory management.  They are deprecated and not present on the arm64 or riscv
     architectures.  The mmap(2) interface should be used to allocate pages instead.

     The brk() and sbrk() functions are used to change the amount of memory allocated in a
     process's data segment.  They do this by moving the location of the “break”.  The break is
     the first address after the end of the process's uninitialized data segment (also known as
     the “BSS”).

     The brk() function sets the break to addr.

     The sbrk() function raises the break by incr bytes, thus allocating at least incr bytes of
     new memory in the data segment.  If incr is negative, the break is lowered by incr bytes.

NOTES

     While the actual process data segment size maintained by the kernel will only grow or shrink
     in page sizes, these functions allow setting the break to unaligned values (i.e., it may
     point to any address inside the last page of the data segment).

     The current value of the program break may be determined by calling sbrk(0).  See also
     end(3).

     The getrlimit(2) system call may be used to determine the maximum permissible size of the
     data segment.  It will not be possible to set the break beyond “etext + rlim.rlim_max” where
     the rlim.rlim_max value is returned from a call to getrlimit(RLIMIT_DATA, &rlim).  (See
     end(3) for the definition of etext).

RETURN VALUES

     The brk() function returns the value 0 if successful; otherwise the value -1 is returned and
     the global variable errno is set to indicate the error.

     The sbrk() function returns the prior break value if successful; otherwise the value (void
     *)-1 is returned and the global variable errno is set to indicate the error.

ERRORS

     The brk() and sbrk() functions will fail if:

     [EINVAL]           The requested break value was beyond the beginning of the data segment.

     [ENOMEM]           The data segment size limit, as set by setrlimit(2), was exceeded.

     [ENOMEM]           Insufficient space existed in the swap area to support the expansion of
                        the data segment.

SEE ALSO

     execve(2), getrlimit(2), mmap(2), end(3), free(3), malloc(3)

HISTORY

     The brk() function appeared in Version 7 AT&T UNIX.  FreeBSD 11.0 introduced the arm64 and
     riscv architectures which do not support brk() or sbrk().

BUGS

     Mixing brk() or sbrk() with malloc(3), free(3), or similar functions will result in non-
     portable program behavior.

     Setting the break may fail due to a temporary lack of swap space.  It is not possible to
     distinguish this from a failure caused by exceeding the maximum size of the data segment
     without consulting getrlimit(2).

     sbrk() is sometimes used to monitor heap use by calling with an argument of 0.  The result
     is unlikely to reflect actual utilization in combination with an mmap(2) based malloc.

     brk() and sbrk() are not thread-safe.